Surface acoustic wave devices have excellent frequency selectivity, and therefore have earned wide acceptance for use in filters for commercial television receivers. Such filters commonly employ at least two acoustically coupled interdigital transducers mounted upon a common piezoelectric substrate, one for sending an acoustic signal and another for receiving it. Often the two transducers are located in different acoustic track areas on the surface of the substrate, and a multistrip coupler is employed to divert the signal from track to track, so as to avoid detecting bulk reflections traveling through the depth of the substrate. Otherwise such reflections would appear in the output of the transducer, and degrade its performance.
But even after the bulk mode reflections have been eliminated from the filter output, there is a still a problem with surface mode acoustic reflections. Part of the signal emitted by the sending transducer is reflected by the receiving transducer, and thus returns to the sending transducer, where it is again partly reflected. The second reflection travels forward to the receiving transducer, arriving out of phase. This so-called triple-transit produces troublesome ghost images on the screen of a TV receiver. As a result, numerous stratagems have been evolved to deal with the problem of unwanted surface acoustic wave reflections, and a large literature has accumulated on the subject of compensating for such reflections.
The stratagems adopted by the prior art have been successful to a point, but the problem has not been entirely eliminated. The underlying concept in most of the corrective schemes is to generate two reflections which are out of phase with each other, and so tend more or less to cancel each other. Sometimes the phase difference is obtained by electrically connecting reflecting elements in phase-opposition, while in other instances it is the spacing of these elements from each other which causes their reflections to be out of phase.
One scheme of the latter variety which has been almost universally adopted is the use of "split-connected" finger structures in which each finger is subdivided into two electrically interconnected halves spaced a quarter wavelength apart, so that they provide two separate reflections which are opposite in phase and equal in magnitude, and thus substantially cancel each other. Even the use of split-connected fingers, however, leaves higher order reflections to be dealt with by other means.
The present invention is aimed at compensating or cancelling those higher order reflections which continue to be a problem even in transducers which employ the split-finger design.
One prior art approach, described in U.S. Pat. No. 4,205,280, uses an electrically induced phase difference. In that patent an interdigital transducer has a finger which comprises an electrically isolated element and an electrically connected element colinear therewith. The electrical differences between these two elements cause their respective reflections to be out of phase with each other, despite the fact that their colinear location would otherwise cause the reflections to be in phase.
The present invention is superficially similar to the structure of the prior patent, in that it dispenses with a portion of the end finger at one end of the transducer, but in this case that finger portion is eliminated entirely, instead of being merely electrically isolated and remaining present as a reflecting element. As a result, the mode of operation is entirely dissimilar and the degree of reflection compensation achieved is substantially greater.
According to this invention, there is provision for electrically connecting the end finger to a bus bar. The end finger is substantially a quarter of an acoustic wavelength out of phase with the nearest interior finger with respect to an incident wave at the operating frequency, so that the acoustic reflections of the end finger and the nearest interior finger are in phase-opposition to each other. In addition, the end finger spans one half of the transducer aperture, and is arranged to permit unobstructed acoustic reflection from the nearest interior finger over the other half of the aperture at one of the ends of the transducer, so that the resulting phase-opposed reflections are equal in amplitude in order to achieve mutual cancellation.
The reflection-compensating finger employed herein, unlike that in the cited patent, is not electrically isolated. On the contrary, it is tied to at least one of the bus bars. Moreover that finger, unlike the colinear isolated and connected reflecting elements in the cited patent, covers only half of the transducer aperture, leaving the neighboring finger "bare" over the remaining half of the aperture, so that the two halves of the transducer reflect out-of-phase waves, and also so that these reflected waves are of substantially equal amplitude for near total cancellation.
The most fundamental difference, however, between the design of this invention and that of the prior art is only evident at the deepest level of their respective theories of operation. There are several underlying mechanisms at work in the production of surface acoustic reflections from transducers. One of these is mass loading; i.e. the inertial load which the mass of the conductive finger material imposes upon the surface layer of the piezoelectric substrate. A second mechanism is the local short-circuiting effect which the conductive elements have upon the voltages piezoelectrically induced in the substrate material. A third reflection mechanism is due to the interconnection of the two halves of each split finger by the bus bars, as a result of which each split finger becomes a short-circuited current loop, and that disturbs an incoming acoustic wave sufficiently to cause reflections.
In the interior of the transducer each of these reflection-producing mechanisms is opposed by an inherent compensation effect: for each element which produces a reflection, one can find an adjoining element which produces an equal and opposite reflection and therefore theoretically (i.e., when the transducer as a whole is short-circuited) cancels the reflection from the other element. That is not true, however at the ends of the transducer, where the acoustic signal enters and exits. Thus, each of these underlying reflection mechanisms described above is effective in producing reflections primarily at the entrance and exit ends of the transducer.
Therefore, when an acoustic signal wavefront traverses a transducer there is a first transient reflection when the signal hits the first finger (for the first two of the reflection mechanisms described above) or the first finger pair (for the third mechanism described) at the entrance end of the transducer; and when it exits, there is a second, similar transient reflection when the signal hits the last finger or finger pair. It is only when the wavefront is traversing the interior of the transducer structure that the reflection-compensating mechanisms are effective for the three reflecion mechanisms described.
It is an objective of this invention to compensate for these entrance and exit end transients. The prior art structure described herein is capable of doing so only with respect to the reflections produced by the third mechanism, but the present invention is capable of doing so with respect to all three of the described mechanisms, with a resulting improvement in echo suppression of almost an order of magnitude.
These and other features, objects and advantages of the invention will now be more fully described in connection with a particular embodiment. This embodiment serves to illustrate the invention, but the invention is not limited thereto. The detailed description of this illustrative embodiment is intended to be read in conjunction with the following drawings, in which like reference characters refer to like elements throughout the several figures: